Note: Aves, or birds, have a complete double circulation,
oviparous, reproduction, front limbs peculiarly
modified as wings; and they bear feathers. All existing
birds have a horny beak, without teeth; but some
Mesozoic fossil birds (Odontornithes) had conical teeth
inserted in both jaws. The principal groups are:
{Carinat[ae]}, including all existing flying birds;
{Ratit[ae]}, including the ostrich and allies, the
apteryx, and the extinct moas; {Odontornithes}, or
fossil birds with teeth.
Note: The ordinary birds are classified largely by the
structure of the beak and feet, which are in direct
relation to their habits. See {Beak}, {Bird},
{Odontonithes}.